This invention relates to sucker rod constructions and more particularly to fiberglass sucker rods. Sucker rods are joined together to form a sucker rod string which connects a sub-surface well pump to a surface pumping unit. This string imparts a reciprocating pumping motion to the sub-surface well pump. Although the sucker rods were generally for decades constructed from steel, it is now known to construct them with a fiberglass rod portion. An example of such a fiberglass sucker rod construction is found in U.S. Pat. No. 4,360,288, the contents of which are hereby incorporated in their entirety by reference.
In the early 1970s, a solution was sought for the chemical and stress corrosion problems related to the use of steel sucker rods. It was found that, in addition to significantly reducing corrosion problems of steel sucker rods, the use of fiberglass rods resulted, in many cases, in increased production. The lighter weight of the rods cuts the pumping unit loads significantly and the reduced pumping unit load allows the pumping unit to operate faster. Also, the elasticity of the prestressed fiberglass rods string generates significant production increases due to the "overtravel." A weight is attached on the bottom of the fiberglass sucker rods, such as steel sucker rods or sinker bars, and this weight causes a higher upstroke relative to the surface stroke and a lower downstroke, that is, overtravel. This provides for a maximum overtravel whereby the pump stroke generates a greater production. The lighter fiberglass rods also allows for a longer string to be used and thus deeper wells to be dug.
Although the fiberglass sucker rods have resulted in greatly increased production and associated reduced pumping costs, premature fatigue failures has been a problem. These premature fatigue failures have occurred both in field applications and in cyclic duty tests of sample rods. It has also been experienced that the ends of the connector members would cut into the fiberglass rods thereby cutting the fibers and weakening the rod.